1. Field of the Invention
The present invention relates to a hybrid vehicle which can use an internal combustion engine and an electric motor as a power source at the same time. More specifically, the invention relates to an improvement of a drive unit for the hybrid vehicle.
2. Description of the Prior Art
In recent years, various hybrid vehicles have been proposed which can decrease the magnitude of noise and the quantity of exhaust gas by using an internal combustion engine as a power source when a high load is applied, and an electric motor as a power source when an intermediate or low load is applied.
For example, Japanese Patent Laid-Open No. 5-50865 discloses a hybrid vehicle in which an electric motor for driving the vehicle is installed in a transmission.
On the other hand, Japanese Patent Publication No. 40-29642 and Japanese Utility Model Publication No. 50-16340 disclose hybrid vehicles in which an electric motor is mounted on on intermediate portion of a propeller shaft extending between a transmission connected to an internal combustion engine and a differential gear for distributing a driving force to driving wheels.
However, the hybrid vehicles disclosed in the aforementioned Japanese Patent Laid-Open No. 5-50865 can not directly use an existing transmission designed for an automotive vehicle which uses an internal combustion engine as an exclusive power source, since the electric motor is built in the transmission. Therefore, it is required to greatly change the design of the transmission, so that a great deal of capital investment is required. In addition, the general-purpose of the electric motor itself built in the transmission is decreased by the restrictions due to the size and layout of the transmission, so as to increase the cost of the electric motor itself. Moreover, in this hybrid vehicle, the electric motor rotates even if the engine is used as the power source, so that useless energy is consumed by the rotation of the electric motor to decrease the fuel consumption.
In addition, in the hybrid vehicles described in the aforementioned Japanese Patent Publication No. 49-29642 and Japanese Utility Model Publication No. 50-16340, only primary driving wheels are driven, and these publications fail to teach a driving unit for a hybrid vehicle in which all of four wheels are driven.